Humidifiers for hot air furnaces and space heating systems are typically comprised of a housing having an air inlet and an air outlet for passing space heating air from the furnace through the housing and over and/or through a water fed evaporator in the housing for moistening the heating air passing through the housing and to the space heating system. The structure, mode of operation and beneficial effects of hot air furnace humidifiers are well known.
A typical bypass humidifier to be used in association with a hot air furnace includes warm or hot air supply ducting, cool or cold air return ducting and an internal blower for forcing heated air from the furnace through the ducting. The humidified air is forced to a space to be heated. Cool air from the space is pulled or drawn through the ducting back to the furnace to be reheated and re-circulated through the space. A bypass humidifier has an inlet in its back wall connected in fluid communication with one or the other of the ducts, usually the hot air supply ducting, and an outlet connected in fluid (gaseous) communication with the other of the ducts, usually the cool air return ducting. Due to the differential between the relatively higher pressure airflow in the supply ducting and the relatively lower pressure airflow in the ducting, air is induced to flow from the supply ducting through the humidifier to the return ducting, thereby causing air to flow over and/or through an evaporator unit in the housing to moisturize, i.e., humidify, the air flowing through the space heating system. A damper installed at the outlet of the humidifier controls the amount of air passing through the humidifier and thus the amount of moisturized air delivered to the space to be heated.
U.S. Pat. No. 5,368,784 suggests a method of improving air flow through the humidification system by placing a scoop inside the air duct so that pressure from the flowing air can be harnessed to push dry air through the humidifier. Air is forced into the humidification system, through an evaporative element, then turned 180° to exit back through the evaporative element and exits via an outlet back into the air duct. This improvement provides continuous humidification and requires only a single opening in the air duct for installation, it is not an efficient humidifier. Passing the humidified air back through the evaporative element actually reduces the amount of moisture that can be held by the humidified air. While absorbing moisture on the first pass through the evaporative element, heat from the hot air is used to generate the latent heat of vaporization of the water. The hot air is further cooled as it continues through the housing and reverses direction. Therefore, as it passes through the evaporative element just prior to exiting the humidifier, the air is cooler than it was as it passed through the evaporative element the first time. Since the air has cooled, it has less capacity to hold moisture, and is thus likely to deposit water in the evaporative element rather than absorb it.
Another disadvantage of this humidifier is that it utilizes only about half of the evaporative element to put water into the air. The hot air comes in the scoop and is directed only toward the bottom half of the evaporative pad. After reversing direction in the space between the pad and the cover, the air flows through the top half of the evaporative pad. Since the top half of the evaporative element is substantially ineffective for putting water into the air, only about half of the surface area of the evaporative element is effectively being used.